If you missed your chance to get your mitts on Portal for free then you really ought to consider taking Steam up on this offer.
As well as being incredibly fun and addictive to play, Portal is one of the smartest and wittiest games I have ever played. It's packed with awesome little quirks (the ending theme alone is enough to justify a download at this price!)
If Portal was a person it would have me swooning in a dribbling puddle on the floor.
The game can be a little tricky to get your head around at first but as soon as you manage to click your mind into "Portal mode" you'll be zipping through dimensions with delight and probably find yourself continuing to think in portals even when you're far and away from your PC. The script is fantastic and is likely to have you laughing out loud on more than one occasion, particularly considering the perfect delivery from the inspired computer voice which reads it. The puzzles themselves are often ingenious and I defy you to come away and not wish fervently that you had your very own real portal gun (or a weighted companion cube to shower with love).
The only negative aspect to Portal that I can think of is its brevity; it is very short. It's unlikely to take you more than a few hours to hammer through, even if you include the special unlockable levels. Unless you're really terrible at it that is. However, considering the fact that it was originally released as little more than a bonus game on the Orange Box and is now being offered for the price of a chocolate bar and a packet of crisps, it's hard to feel too put out by its length.
Thanks to callmecheez at Hotukdeals!